The present invention generally relates to an insitu, nondestructive technique for determining the amount of fatigue and stress corrosion damage occurring in a material before the onset of a propagating crack. More particularly, the method comprises the utilization of x-ray diffraction techniques to determine the average excess dislocation density over a certain depth range in the surface layer of the material and comparing such value with a predetermined critical value of the excess dislocation density for the material.
In general, fatigue is a term used to describe the behavior of materials under repeated cycles of stress or strain which cause a deterioration of the material that results in progressive cracking and failure of such material. Conventional fatigue testing machines, as disclosed in U.S. Pat. No. 2,729,096 for example, are utilized to apply alternating stresses and displacements to a test specimen until the specimen fails after a number of repetitive loading cycles. Such fatigue machines are normally designed to apply either an axial load, bending or flexural loads, torsional loads or a combination thereof, wherein the test specimen in such machines are normally loaded by applying either a constant deflection or a constant load thereto. However, since the test specimens utilized in fatigue testing machines are normally simplified models of the actual structural or machine element and since the stress conditions applied by the fatigue testing machines do not approximate the stress-time or strain-time history of the element, the fatigue strength determined thereby is usually not a close approximation of the actual fatigue strength of the element.
X-ray diffraction techniques have been developed to determine various physical properties of materials, as exemplified by U.S. Pat. Nos. 3,934,138; 4,095,103; 4,125,771 and 4,128,762 which are directed to methods of determining the residual stress in materials. However, past attempts to use x-ray diffraction line broadening to measure fatigue damage were unsuccessful, as noted for example by C. S. Barrett on pages 337-338 of Structure of Metals; Crystallographic Methods, Principles and Data, published by McGraw-Hill in 1943. In these prior investigations it was reported that the x-ray diffraction lines of the immediate surface broadened as the material was initially fatigued, but that after a small fraction (eg. about 15%) of the fatigue life of the material, the diffraction line broadening tended to remain virtually unaltered, both in extent and intensity, throughout the remainder of the fatigue life of the material. Since these prior investigations assumed that the structural changes in the interior or bulk region and at the surface were the same, the fatigue damage and the remaining fatigue life of the materials were inaccurate. In actuality, it was discovered, as noted in the Determination of Prefracture Fatigue Damage, by I. Kramer, S. Weissmann and R. Pangborn and published by the David W. Taylor Naval Ship Research and Development Center (DTNSRDC), Bethesda, Md., in January 1980, (Report DTNSRDC 80/006) and herein incorporated by reference, that the structural changes in the bulk region and at the surface were different. For example, it was found that the surface of the material work hardened, as measured by the increase in the dislocation density, to a much greater extent than the interior or bulk region.